A unique characteristic of nuclear energy is that used fuel may be separated from other components and reused as new fuel. For instance, the nuclear materials contained in a spent rod from a nuclear power plant can be reprocessed and reused to produce new fuel rods. Practically all nuclear materials, including uranium and plutonium, can be reprocessed in this manner.
Fuel elements, including fuel rods in nuclear reactors, become unusable not so much on account of actual depletion of the fissionable fuel values, but because of the accumulation within the element of fission products. These fission products can interfere with the neutron flux within the reactor. Consequently, fuel elements are withdrawn from the reactor long before the fuel values are anywhere near to being completely consumed. The withdrawn or used nuclear fuel (sometimes referred to as spent fuel rods) have significant fuel value. At the same time, it is desirable to recover the valuable by-products of reactor operation, the transmutation products such as plutonium, which is a fissionable fuel, and certain isotopes of the fission products which are useful in many different fields and have many different applications.
Many research reactor fuel assemblies or fuel plates contain a nuclear material in combination with aluminum, such as a uranium-aluminum alloy or a uranium aluminide dispersed in a continuous aluminum phase. Aluminum is also widely used as a fuel element cladding material because it has a relatively low neutron absorption cross-section and has excellent physical and chemical properties. One type of aluminum used as a cladding material includes 1100 aluminum. Other alloys include 6061 and 6063.
A conventional process for recovering nuclear materials from used nuclear fuel is a dissolution process during which the aluminum material is dissolved. In one embodiment, the process for recovering fissionable materials is an aqueous process during which the fuel elements are dissolved in an acidic solution. Fuel elements containing an aluminum-uranium alloy contained in aluminum cladding, for instance, may be dissolved in a mercury-catalyzed, nitric acid flowsheet. After the fuel is dissolved in the solution, the uranium can be recovered from the aluminum and fission products. The dissolution process must be carefully controlled to ensure that the used nuclear fuel dissolves at an acceptable rate without producing unacceptable amounts of off-gas.
The off-gas generation rate during nuclear fuel dissolution changes depending upon many factors. Thus, the off-gas generation rate is never constant. Off-gases that are produced include nitrogen oxides, hydrogen gas, in addition to volatile fission product gases, such as krypton, xenon and iodine vapor. The mechanisms that impact off-gas concentrations and species that produce the above gases during the course of dissolution are complex and are not well understood. Spikes in the generation of off-gases, however, can produce significant amounts of hydrogen gas which may drift above safety levels in the processing plant.
In the past, various processes have been proposed in order to control the generation of off-gases. For instance, U.S. Pat. No. 3,119,658 to Schulz, which is incorporated herein by reference, suggests that the rate of reactions during dissolution can be controlled by adding small amounts of nickelous nitrate or nickel II ion. The process proposed in the '658 patent, however, has had limited success.
Consequently, a need exists for a method or process of decreasing the off-gas generation rate or hydrogen gas concentration in the off-gas during used nuclear fuel dissolution processes. Controlling the off-gas rate and/or the amount of hydrogen gas being produced can significantly increase throughput and efficiency.